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United States Patent |
5,661,743
|
Nagai
|
August 26, 1997
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Semiconductor laser
Abstract
A semiconductor laser includes an n type GaAs semiconductor substrate, an n
type AlGaAs cladding layer, an active layer producing light having a
wavelength equal to or larger than 900 nm, a p type AlGaAs cladding layer,
and an n type AlGaAs current blocking layer having a current concentrating
structure. The n type AlGaAs current blocking layer comprises Al.sub.x
Ga.sub.1-x As having an Al composition ratio x smaller than an Al
composition ratio of the p type AlGaAs cladding layer, and is doped with
Si to a concentration equal to or larger than 1.times.10.sup.19 cm.sup.-3.
Therefore, since more V.sub.III -Si.sub.Ga complexes C are produced in the
AlGaAs current blocking layer than those produced when the current
blocking layer is GaAs and absorption of light having a wavelength of
0.9.about.1.2 .mu.m is promoted, generation of higher-order modes is
reliably suppressed and laser light having a fundamental mode is stably
produced. Consequently, a semiconductor laser for exciting a fiber
amplifier and excellent as a pumping light source of an optical fiber is
realized.
Inventors:
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Nagai; Yutaka (Tokyo, JP)
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Assignee:
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Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
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Appl. No.:
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691303 |
Filed:
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August 2, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
372/46.01; 372/45.013; 372/50.22 |
Intern'l Class: |
H01S 003/19 |
Field of Search: |
372/46,45,44
|
References Cited
U.S. Patent Documents
5325385 | Jun., 1994 | Kasukawa et al. | 372/46.
|
5351258 | Sep., 1994 | Okumura et al. | 372/46.
|
Foreign Patent Documents |
2106085 | Apr., 1990 | JP.
| |
521902 | Jan., 1993 | JP.
| |
Other References
Oh-hori et al., "Donor-Cation Vacancy Complex In Si-Doped AlGaAs Grown By
Metalorganic Chemical Vapor Deposition", Journal of Applied Physics, vol.
61, No. 9, May 1987, pp. 4603-4605.
|
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Leydig, Voit & Mayer
Claims
What is claimed is:
1. A semiconductor laser including:
a n type GaAs semiconductor substrate;
an n type AlGaAs cladding layer;
an active layer producing light having a wavelength no shorter than 900 nm;
a p type AlGaAs cladding layer having an Al composition ratio; and
an n type AlGaAs current blocking layer having a current concentrating
structure, the n type AlGaAs current blocking layer comprising Al.sub.x
Ga.sub.1-x Al having an Al composition ratio x smaller than the Al
composition ratio of the p type AlGaAs cladding layer, the current
blocking layer being doped with Si in a concentration no smaller than
1.times.10.sup.19 cm.sup.-3, and the current concentrating structure being
a ridge structure comprising the p type AlGaAs cladding layer and having a
striped-shaped ridge structure, and the n type AlGaAs current blocking
layer disposed on both sides of the ridge of the p-type AlGaAs cladding
layer.
2. The semiconductor laser of claim 1 wherein the n type AlGaAs current
blocking layer has an Al composition ratio x larger than 0 and no larger
than 0.3.
3. A semiconductor laser including;
a n type GaAs semiconductor substrate;
an n type AlGaAs cladding layer;
an active layer producing light having shorter than 900 nm;
a p type AlGaAs cladding layer having an Al composition ratio; and
an n type AlGaAs current blocking layer having a current concentrating
structure, the n type AlGaAs current blocking layer comprising Al.sub.x
Ga.sub.1-x Al having an Al composition ratio x smaller than the Al
composition ratio of the p type AlGaAs cladding layer, the current
blocking layer being doped with Si in a concentration no smaller than
1.times.10.sup.19 cm.sup.-3, and the current concentrating structure being
an SAS (self-aligned structure) structure comprising the n type AlGaAs
current blocking layer having a central stripe groove, and the p type GaAs
cladding layer disposed on the n type AlGaAs current blocking layer
including the stripe groove.
4. The semiconductor laser of claim 3 wherein the n type AlGaAs current
blocking layer has an Al composition ratio x larger than 0 and no larger
than 0.3.
Description
FIELD OF THE INVENTION
The present invention relates to a semiconductor laser and a fabricating
method thereof and, more particularly, to a semiconductor laser for
exciting a fiber amplifier doped with Er (erbium) or Pr (praseodymium) and
a fabricating method thereof.
BACKGROUND OF THE INVENTION
FIG. 8 is a perspective view illustrating a prior art semiconductor laser
including a ridge structure (hereinafter referred to as a ridge type
semiconductor laser), and FIGS. 9(a)-9(e) are cross-sectional views
illustrating process steps in a method of fabricating the semiconductor
laser. FIG. 10 is a diagram illustrating a refractive index profile in a
ridge region of the semiconductor laser.
In FIG. 8, reference numeral 1 designates an n type GaAs semiconductor
substrate. An n type AlGaAs cladding layer 2 comprising Al.sub.0.5
Ga.sub.0.5 As is disposed on the n type GaAs semiconductor substrate 1. A
quantum-well active layer 3 comprising undoped InGaAs is disposed on the n
type AlGaAs cladding layer 2. A p type AlGaAs first cladding layer 4
comprising Al.sub.0.5 Ga.sub.0.5 As is disposed on the quantum-well active
layer 3. A p type AlGaAs etch stopping layer 5 comprising Al.sub.0.7
Ga.sub.0.3 As is disposed on the p type AlGaAs first cladding layer 4. A p
type AlGaAs second cladding layer 6 comprising Al.sub.0.5 Ga.sub.0.5 As
and a p type GaAs first contact layer 7 are successively disposed on the p
type AlGaAs etch stopping layer 5, and have a stripe-shaped ridge
structure. Reference numeral 12 designates a ridge waveguide, and the
ridge waveguide 12 has a width Wb in a range of 1.about.1.5 .mu.m at a
boundary between the ridge waveguide and the p type AlGaAs etch stopping
layer 5. N type AlGaAs current blocking layers 14 comprising Al.sub.0.7
Ga.sub.0.3 As are disposed on the p type AlGaAs etch stopping layer 5 at
both sides of the ridge structure comprising the p type AlGaAs second
cladding layer 6 and the p type GaAs first contact layer 7. A p type GaAs
second contact layer 9 is disposed on the ridge structure and on the n
type AlGaAs current blocking layers 14. A p side electrode 10 is disposed
on a rear surface of the n type GaAs semiconductor substrate 1, and an n
side electrode 11 is disposed on the p type GaAs second contact layer 9.
A description is given of the fabricating method.
Initially, as shown in FIG. 9(a), the n type AlGaAs cladding layer 2, the
InGaAs quantum-well active layer 3, the p type AlGaAs first cladding layer
4, the p type AlGaAs etch stopping layer 5, the p type AlGaAs second
cladding layer 6, and the p type GaAs first contact layer 7 are
successively epitaxially grown on the n type GaAs semiconductor substrate
1, preferably by metal organic chemical vapor deposition (MOCVD).
Next, as shown in FIG. 9(b), a stripe-shaped insulating film 13 comprising
Si.sub.3 N.sub.4 or SiO.sub.2 is deposited on the p type GaAs first
contact layer 7. The insulating film 13 serves as a mask for ridge
etching. In the step of FIG. 9(c), using the insulating film 13 as a mask,
the p type AlGaAs second cladding layer 6 and the p type GaAs first
contact layer 7 are selectively etched to form a stripe-shaped ridge
structure. The selective etching is performed using an etchant, such as a
solution of tartaric acid and hydrogen peroxide, that does not etch the p
type AlGaAs etch stopping layer 5 but etches the p type AlGaAs second
cladding layer 6 and the p type GaAs first contact layer 7. Therefore, the
ridge structure comprising the p type AlGaAs second cladding layer 6 and
the p type GaAs first contact layer 7 can be formed with good
reproducibility.
Thereafter, as shown in FIG. 9(d), the n type AlGaAs current blocking layer
14 is grown on both sides of the ridge structure to bury portions of the p
type AlGaAs second cladding layer 6 and the p type GaAs first contact
layer 7 which are removed by the etching. Since the insulating film 13
serves as a mask during the crystal growth, the n type AlGaAs current
blocking layer 14 is not grown on the ridge structure.
In the step of FIG. 9(e), after removing the insulating film 13 by wet
etching or dry etching, the p type GaAs second contact layer 9 is grown on
the entire surface. The n side electrode 10 is formed on the n type GaAs
semiconductor substrate 1 and the p side electrode 11 is formed on the p
type GaAs second contact layer 9, resulting in the semiconductor laser
shown in FIG. 8.
A description is given of the operation.
When a voltage is applied across the electrodes so that the p side
electrode 11 is plus and the n side electrode 10 is minus, holes are
injected into the quantum-well active layer 3 through the p type GaAs
second contact layer 9, the p type GaAs first contact layer 7, the p type
AlGaAs second cladding layer 6, the p type AlGaAs etch stopping layer 5,
and the p type AlGaAs first cladding layer 4 and electrons are injected
into the quantum-well active layer 3 through the n type GaAs semiconductor
substrate 1 and the n type AlGaAs cladding layer 2. Then, the electrons
and holes recombine in the quantum-well active layer 3 and stimulated
emission light is generated therein. When the quantity of carriers
(electrons and holes) which are injected into the active layer is
sufficiently large and light exceeding the waveguide loss is produced,
laser oscillation occurs.
In a region in the vicinity of the n type AlGaAs current blocking layer 14
except the stripe-shaped ridge region, pn junctions are formed at the
interfaces between the n type AlGaAs current blocking layer 14 and the p
type AlGaAs first cladding layer 4 and between the n type AlGaAs current
blocking layer 14 and the p type GaAs second contact layer 9. Therefore,
even when a voltage is applied so that the p side electrode 11 is plus,
the region in the vicinity of the n type AlGaAs current blocking layer 14
is reversely biased because of the p-n-p junction, so that no current
flows through this region. That is, the n type AlGaAs current blocking
layer 14 blocks current flow. Consequently, a current flows only through
the ridge region and is concentrated only in a central portion of the
quantum-well active layer 3 just below the ridge region, whereby a current
density sufficient to produce laser oscillation is achieved.
A description is given of a waveguide structure for laser light in the
prior art semiconductor laser.
Generally, in a semiconductor laser, various structural devices have been
used in order to realize a unimodal laser beam having a fundamental
transverse mode. More specifically, a semiconductor laser has a waveguide
structure comprising a double heterostructure in a direction perpendicular
to a pn junction, i.e., in a direction perpendicular to a substrate
surface, whereby a laser beam having a fundamental transverse mode is
produced stably. Therefore, in the prior art semiconductor laser, since
the AlGaAs cladding layers 2, 4, and 6 have respective refractive indices
smaller than the refractive index of the InGaAs quantum-well active layer
3, the laser light is guided in the quantum-well active layer 3 having a
relatively large refractive index. This is because light has the property
of passing through a medium having a large refractive index.
In addition, in a ridge type semiconductor laser, a ridge waveguide has a
refractive index profile as shown in FIG. 10 in a direction parallel to
the pn junction, i.e., in a direction parallel to the substrate surface,
whereby a laser beam having a fundamental transverse mode is produced.
Therefore, in the prior art semiconductor laser, since the p type AlGaAs
cladding layer 6 in the ridge waveguide 12 has a refractive index larger
than that of the n type AlGaAs current blocking layer 14, the laser light
is guided along the ridge waveguide 12. Consequently, the horizontal
transverse mode that is an important characteristic of the semiconductor
laser becomes stable and unimodal.
As described above, the prior art semiconductor laser shown in FIG. 8
guides the light, utilizing the difference in refractive index in the
ridge structure. In this semiconductor laser, however, in view of mode
control, the waveguide width Wb at the boundary between the ridge
waveguide 12 and the etch stopping layer 5 must be in a range of
1.about.1.5 .mu.m. It is probable that a semiconductor laser having a
waveguide width Wb larger than 1.5 .mu.m produces higher-order modes
higher than or equal to the second-order mode as well as a fundamental
mode, and the semiconductor laser producing the higher-order modes shows a
nonlinear characteristic called a "kink" in the current-light output
characteristic, which adversely affects the laser in practical use.
Further, when the semiconductor laser is used to output the laser beam to
a fiber, generation of a multimodal laser beam having higher-order modes
would exceptionally lower the coupling efficiency between the fiber and
the semiconductor laser. Consequently, in order to fabricate this kind of
semiconductor laser stably, it is desirable that the waveguide width Wb
should be about 1 .mu.m, considering its margin.
However, when the waveguide width Wb is small, current density during
operation becomes extremely high, or optical density at the semiconductor
laser facet becomes high. Generally, reliability of a semiconductor laser
is reduced by internal deterioration and facet destruction. The internal
deterioration is caused by an increase of dislocations in an active layer
at high current density, and facet destruction is caused by melting of
facet portions resulting from high optical density. Therefore, in the
prior art semiconductor laser shown in FIG. 8, since the waveguide width
Wb is small, i.e., 1.about.1.5 .mu.m, internal deterioration under high
current density and facet destruction resulting from high optical density
occur, so that reliability of the semiconductor laser is extremely
reduced.
In addition, the half-power angular width of the horizontal transverse mode
depends on the waveguide width Wb. As the waveguide width Wb is reduced,
the half-power angular width varies greatly even when the width Wb varies
slightly. Accordingly, the half-power angular width of the horizontal
transverse mode varies widely as the waveguide width Wb is reduced,
whereby the fabrication yield of the semiconductor laser is reduced.
Consequently, in the prior art semiconductor laser shown in FIG. 8, since
the waveguide width Wb is small, i.e., 1.about.1.5 .mu.m, the half-power
angular width of the horizontal transverse mode varies widely, whereby the
fabrication yield of the semiconductor laser is reduced.
Further, the prior art semiconductor laser shown in FIG. 8 includes the
current blocking layer 14 comprising Al.sub.0.7 Ga.sub.0.3 As, and has a
structure utilizing the difference in refractive index in the ridge
structure, i.e., a refractive index type structure. Therefore, the light
extending to both sides of the ridge structure is not absorbed, so that it
is probable to produce higher-order modes having peaks at the end portions
of the ridge waveguide, whereby the coupling efficiency between the
semiconductor laser and the optical fiber is lowered.
Patent Application No. Hei. 7-178759 discloses another prior art ridge type
semiconductor laser in which a quantum-well active layer emitting laser
light having a wavelength of 0.98 .mu.m and a ridge waveguide comprising a
p type Al.sub.0.5 Ga.sub.0.5 As cladding layer having a width of 2.about.5
.mu.m are disposed on an n type GaAs semiconductor substrate, and
Al.sub.0.7 Ga.sub.0.3 As current blocking layers doped with Er as a metal
for absorbing the laser light having a wavelength of 0.98 .mu.m are
disposed at both sides of the ridge waveguide.
The above-described prior art semiconductor laser includes a loss guide
type structure in which the Er-doped Al.sub.0.7 Ga.sub.0.3 As current
blocking layer is used in place of the n type AlGaAs current blocking
layer 14 in the prior art semiconductor laser shown in FIG. 8 and the
laser light is absorbed by this Er-doped AlGaAs current blocking layer.
More specifically, in this prior art semiconductor laser, Er ions in the
current blocking layer absorb the laser light having a wavelength of 0.98
.mu.m which is emitted from the quantum-well active layer just below the
ridge waveguide. The absorption of the laser light by the current blocking
layer is promoted more at the end portions than at the center portion of
the ridge waveguide, whereby gains of higher-order modes having the peaks
at the end portions of the ridge waveguide are reduced. Consequently,
according to this prior art semiconductor laser, even when the waveguide
width is 2.about.5 .mu.m, a unimodal laser beam having a fundamental
transverse mode is produced stably.
FIG. 11 is a cross-sectional view illustrating another prior art
semiconductor laser disclosed in Japanese Published Patent Application No.
Hei. 5-21902. In the figure, reference numeral 101 designates an n type
GaAs substrate. An Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 102, a
double quantum-well active layer 103, a p type Al.sub.0.5 Ga.sub.0.5 As
upper cladding layer 104, and a p type GaAs ohmic contact layer 105 are
successively disposed on the n type GaAs substrate 101. The p type
Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 104 and the p type GaAs
ohmic contact layer 105 have a stripe-shaped ridge structure 106 produced
by selective etching. N type GaAs current blocking layers 111 doped with
silicon (Si) are disposed so as to bury portions of the upper cladding
layer 104 and the ohmic contact layer 105 which are removed by the
etching. An n side electrode 108 is disposed on a rear surface of the GaAs
substrate 101 and a p side electrode 109 is disposed on the ridge
structure and on the current blocking layers 111. The active layer 103 has
a quantum-well structure comprising an Al.sub.0.3 Ga.sub.0.7 As barrier
layer 103c, two GaAs quantum-well layers 103d, and two Al.sub.0.3
Ga.sub.0.7 As guiding layers 103b. The AlGaAs barrier layer 103c is
sandwiched between the two GaAs quantum-well layers 103d, and further, the
GaAs quantum-well layers 103d are sandwiched between the two AlGaAs
Guiding layers 103b.
In the prior art semiconductor laser shown in figure 11, the n type GaAs
current blocking layer 111 is doped with Si to a high concentration, the
current blocking layer 111 has an n type carrier concentration equal to or
larger than 6.times.10.sup.18 cm.sup.-3, and the current blocking layer
111 absorbs light having a wavelength equal to or larger than 900 nm which
travels from the active layer 103 to the n type GaAs current blocking
layer 111 through the upper cladding layer 104 during laser oscillation.
More specifically, according to this prior art semiconductor laser shown
in FIG. 11, since a broad deep level extending over a 900.about.1000 nm
band is formed in the n type GaAs current blocking layer 111, the current
blocking layer 111 absorbs the light having a wavelength equal to or
larger than 900 nm which travels from both sides of the light emitting
portion to the current blocking layer 111 through the upper cladding layer
104, whereby generation of higher-order modes is suppressed and a
fundamental mode is produced.
In the prior art semiconductor laser disclosed by Patent Application No.
Hei. 7-178759, however, when the current blocking layer is doped with Er,
the absorption peak is shifted from 0.98 .mu.m due the narrow absorption
band; Er ions have an absorption band for light having a wavelength of
0.98 .mu.m. Therefore, it is impossible to suppress the generation of the
higher-order modes reliably.
In the prior art semiconductor laser shown in figure 11, the broad deep
level is formed in the vicinity of a 900 .about.1000 nm band in the n type
GaAs current blocking layer 111 which is doped with Si to a high
concentration. However, due to insufficient light absorption, the
generation of the higher-order modes cannot be suppressed reliably.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor laser
producing laser light having a wavelength of 0.9.about.1.2 .mu.m, in which
a unimodal laser beam having a fundamental transverse mode is produced
stably and securely, and a method of fabricating the semiconductor laser.
Other objects and advantages of the present invention will become apparent
from the detailed description given hereinafter; it should be understood,
however, that the detailed description and specific embodiment are given
by way of illustration only, since various changes and modifications
within the scope of the invention will become apparent to the those
skilled in the art from this detailed description.
According to a first aspect of the present invention, a semiconductor laser
includes an n type GaAs semiconductor substrate, an n type AlGaAs cladding
layer, an active layer producing light having a wavelength equal to or
larger than 900 nm, a p type AlGaAs cladding layer, and an n type AlGaAs
current blocking layer having a current concentrating structure. The n
type AlGaAs current blocking layer comprises Al.sub.x Ga.sub.1-x As having
an Al composition ratio x smaller than an Al composition ratio of the p
type AlGaAs cladding layer, and is doped with Si to a concentration equal
to or larger than 1.times.10.sup.19 cm.sup.-3. Therefore, since more
V.sub.III -Si.sub.Ga complexes C are produced in the AlGaAs current
blocking layer than those produced when the current blocking layer
comprises GaAs and absorption of light having a wavelength of
0.9.about.1.2 .mu.m is promoted, generation of higher-order modes is
securely suppressed and laser light having a fundamental mode is stably
produced. Consequently, a semiconductor laser for exciting a fiber
amplifier excellent as a pumping light source of an optical fiber is
realized.
According to a second aspect of the present invention, in the
above-described semiconductor laser, the n type AlGaAs current blocking
layer has an Al composition ratio x larger than 0 and equal to or smaller
than 0.3. Therefore, since more V.sub.III -Si.sub.Ga complexes C are
produced in the AlGaAs current blocking layer than those produced when the
current blocking layer comprises GaAs and absorption of light having a
wavelength of 0.9.about.1.2 .mu.m is promoted, generation of higher-order
modes is securely suppressed and laser light having a fundamental mode is
stably produced. Consequently, a semiconductor laser for exciting a fiber
amplifier excellent as a pumping light source of an optical fiber is
realized.
According to a third aspect of the present invention, in the
above-described semiconductor laser, the current concentrating structure
is a ridge type structure comprising the p type AlGaAs cladding layer
having a stripe-shaped ridge structure, and the n type AlGaAs current
blocking layer disposed on both sides of the p type AlGaAs cladding layer.
Therefore, since more V.sub.III -Si.sub.Ga complexes C are produced in the
AlGaAs current blocking layer and absorption of light having a wavelength
of 0.9.about.1.2 .mu.m is promoted, generation of higher-order modes
having the peaks at end portions of a ridge waveguide is securely
suppressed and laser light having a fundamental mode is stably produced.
Consequently, a semiconductor laser for exciting a fiber amplifier
excellent as a pumping light source of an optical fiber is realized.
According to a fourth aspect of the present invention, in the
above-described semiconductor laser, the current concentrating structure
is an SAS (self-aligned structure) type structure comprising the n type
AlGaAs current blocking layer having a stripe groove in the center, and
the p type AlGaAs cladding layer disposed on the entire surface of the n
type AlGaAs current blocking layer including the stripe groove. Therefore,
since more V.sub.III -Si.sub.Ga complexes C are produced in the AlGaAs
current blocking layer and absorption of light having a wavelength of
0.9.about.1.2 .mu.m is promoted, generation of higher-order modes having
the peaks at end portions of a waveguide is securely suppressed and laser
light having a fundamental mode is stably produced. Consequently, a
semiconductor laser for exciting a fiber amplifier excellent as a pumping
light source of an optical fiber is realized.
According to a fifth aspect of the present invention, a method of
fabricating a semiconductor laser includes successively forming an n type
AlGaAs cladding layer, an active layer, a p type AlGaAs first cladding
layer, a p type AlGaAs etch stopping layer, a p type AlGaAs second
cladding layer, and a p type GaAs first contact layer on an n type GaAs
semiconductor substrate; depositing a stripe-shaped insulating film on the
p type GaAs first contact layer; using the insulating film as a mask,
etching the p type GaAs first contact layer and the p type AlGaAs second
cladding layer selectively with respect to the p type AlGaAs etch stopping
layer using an etchant that does not etch the p type AlGaAs etch stopping
layer but etches the p type GaAs first contact layer and the p type AlGaAs
second cladding layer, thereby forming a stripe-shaped ridge structure
comprising the p type GaAs first contact layer and the p type AlGaAs
second cladding layer; forming an n type AlGaAs current blocking layer
comprising Al.sub.x Ga.sub.1-x As having an Al composition ratio x larger
than 0 and equal to or smaller than 0.3 which is doped with Si to a
concentration equal to or larger than 1.times.10.sup.19 cm.sup.-3 on both
sides of the ridge structure to bury portions of the p type GaAs first
contact layer and the p type AlGaAs second cladding layer which are
removed by the etching; after removing the stripe-shaped insulating film
by etching, forming a p type GaAs second contact layer on the entire
surface; and forming an n side electrode on the rear surface of the n type
GaAs semiconductor substrate and forming a p side electrode on the p type
GaAs second contact layer. Consequently, a ridge type semiconductor laser
emitting unimodal laser light having a stable fundamental transverse mode
is fabricated.
According to a sixth aspect of the present invention, a method of
fabricating a semiconductor laser includes successively forming an n type
AlGaAs cladding layer, an active layer, a p type AlGaAs first cladding
layer, and an n type AlGaAs current blocking layer comprising Al.sub.x
Ga.sub.1-x As having an Al composition ratio x larger than 0 and equal to
or smaller than 0.3 which is doped with Si to a concentration equal to or
larger than 1.times.10.sup.19 cm.sup.-3 on an n type GaAs semiconductor
substrate; depositing an insulating film having a stripe-shaped aperture
on the n type AlGaAs current blocking layer; using the insulating film as
a mask, selectively etching the n type AlGaAs current blocking layer until
a surface of the p type AlGaAs first cladding layer is exposed, thereby
forming a stripe groove in the n type AlGaAs current blocking layer; after
removing the insulating film by etching, successively forming a p type
AlGaAs second cladding layer and a p type GaAs contact layer on the entire
surface of the n type AlGaAs current blocking layer and on the exposed
surface of the p type AlGaAs first cladding layer to bury the stripe
groove in the n type AlGaAs current blocking layer; and forming an n side
electrode on the rear surface of the n type GaAs semiconductor substrate
and forming a p side electrode on the p type GaAs contact layer.
Consequently, an SAS type semiconductor laser emitting unimodal laser
light having a stable fundamental transverse mode is fabricated.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating a semiconductor laser in
accordance with a first embodiment of the present invention.
FIGS. 2(a)-2(e) are cross-sectional views illustrating process steps in a
method of fabricating the semiconductor laser according to the first
embodiment of the invention.
FIG. 3 is a cross-sectional view illustrating an active layer of the
semiconductor laser according to the first embodiment of the invention.
FIG. 4 is a diagram illustrating a molecule structure in a current blocking
layer of the semiconductor laser according to the first embodiment of the
invention.
FIG. 5 is a graph showing Al dependency of an emission peak energy of a
V.sub.III -Si.sub.Ga complex C in Al.sub.x Ga.sub.1-x As having an Si
concentration of about 1.times.10.sup.19 cm.sup.-3.
FIG. 6 is a cross-sectional view illustrating a semiconductor laser in
accordance with a second embodiment of the present invention.
FIGS. 7(a)-7(e) are cross-sectional views illustrating process steps in a
method of fabricating the semiconductor laser according to the second
embodiment of the invention.
FIG. 8 is a perspective view illustrating a prior art semiconductor laser.
FIGS. 9(a)-9(e) are cross-sectional views illustrating process steps in a
method of fabricating the prior art semiconductor laser.
FIG. 10 is a refractive index profile in a direction parallel to a pn
junction, in a ridge structure of the prior art semiconductor laser.
FIG. 11 is a cross-sectional view illustrating another prior art
semiconductor laser.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[Embodiment 1]
FIG. 1 is a perspective view illustrating a semiconductor laser including a
ridge structure (hereinafter referred to as a ridge type semiconductor
laser) according to a first embodiment of the present invention. This
semiconductor laser of the first embodiment is used as a light source for
exciting an Er-doped fiber amplifier. In the figure, reference numeral 1
designates an n type GaAs semiconductor substrate having a thickness of
100 .mu.m and a dopant concentration of 1.about.3.times.10.sup.18
cm.sup.-3. An n type AlGaAs cladding layer 2 comprising Al.sub.0.5
Ga.sub.0.5 As and having a thickness of 2.0 .mu.m and a dopant
concentration of 4.times.10.sup.17 cm.sup.-3 is disposed on the n type
GaAs semiconductor substrate 1. A quantum-well active layer 3 comprising
undoped InGaAs and having a thickness of 1.0 .mu.m is disposed on the n
type AlGaAs cladding layer 2. The structure of the quantum-well active
layer 3 is detailed later. A p type AlGaAs first cladding layer 4
comprising Al.sub.0.5 Ga.sub.0.5 As and having a thickness of
0.1.about.0.3 .mu.m and a dopant concentration of 2.times.10.sup.18
cm.sup.-3 is disposed on the quantum-well active layer 3. A p type AlGaAs
etch stopping layer 5 comprising Al.sub.0.7 Ga.sub.0.3 As and having a
thickness of 20 nm and a dopant concentration of 2.times.10.sup.18
cm.sup.-3 is disposed on the p type AlGaAs first cladding layer 4. A p
type AlGaAs second cladding layer 6 comprising Al.sub.0.5 Ga.sub.0.5 As
and having a thickness of 1.5.about.1.8 .mu.m and a dopant concentration
of 2.times.10.sup.18 cm.sup.-3, and a p type GaAs first contact layer 7
having a thickness of 0.2 .mu.m and a dopant concentration of
2.times.10.sup.19 cm.sup.-3 are successively disposed on the p type AlGaAs
etch stopping layer 5, and have a stripe-shaped ridge structure. Reference
numeral 12 designates a ridge waveguide, and the ridge waveguide 12 has a
width Wb in a range of 2.about.4 .mu.m at a boundary between the ridge
waveguide and the p type AlGaAs etch stopping layer 5. N type AlGaAs
current blocking layers 8 are disposed on the p type AlGaAs etch stopping
layer 5 at both sides of the ridge structure comprising the p type AlGaAs
second cladding layer 6 and the p type GaAs first contact layer 7. The n
type AlGaAs current blocking layers 8 comprise Al.sub.0.2 Ga.sub.0.8 As
doped with Si to a concentration equal to or larger than 1.times.10.sup.19
cm.sup.-3 and have a thickness of 1.5 .mu.m. A p type GaAs second contact
layer 9 having a thickness of 2, .mu.m and a dopant concentration of
2.times.10.sup.19 cm.sup.-3 is disposed on the ridge structure comprising
the p type AlGaAs second cladding layer 6 and the p type GaAs first
contact layer 7 and on the n type AlGaAs current blocking layers 8. A p
side electrode 10 is disposed on a rear surface of the n type GaAs
semiconductor substrate 1, and an n side electrode 11 is disposed on the p
type GaAs second contact layer 9.
The semiconductor laser according to the first embodiment differs from the
prior art semiconductor laser in that the current blocking layer is the n
type AlGaAs layer comprising Al.sub.0.2 Ga.sub.0.8 As having an Al
composition ratio x of 0.2 which is doped with Si to a concentration equal
to or larger than 1.times.10.sup.19 cm.sup.-3. The waveguide structure in
this semiconductor laser is a loss guide type structure that is different
from the refractive index type structure as in the prior art semiconductor
laser shown in FIG. 8.
A description is given of the fabricating method.
FIGS. 2(a)-2(e) are cross-sectional views illustrating process steps in the
fabricating method. Initially, as shown in FIG. 2(a), the n type
Al.sub.0.5 Ga.sub.0.5 As cladding layer 2, the quantum-well active layer
3, the p type Al.sub.0.5 Ga.sub.0.5 As first cladding layer 4, the p type
Al.sub.0.7 Ga.sub.0.3 As etch stopping layer 5, the p type Al.sub.0.5
Ga.sub.0.5 As second cladding layer 6, and the p type GaAs first contact
layer 7 are successively epitaxially grown on the n type GaAs
semiconductor substrate 1, preferably by metal organic chemical vapor
deposition (MOCVD). The growth of the layers except the active layer 3 is
carried out under the conditions of a growth temperature of about
700.degree. C., a V/III ratio of 200, and a growth speed of 1 .mu.m/h.
Next, as shown in FIG. 2(b), a stripe-shaped insulating film 13 comprising
Si.sub.3 N.sub.4 or SiO.sub.2 is deposited on the p type GaAs first
contact layer 7. The insulating film 13 serves as a mask for ridge
etching. In the step of FIG. 2(c), using the insulating film 13 as a mask,
the p type AlGaAs second cladding layer 6 and the p type GaAs first
contact layer 7 are selectively etched to form a stripe-shaped ridge
structure. The etching is performed using an etchant, such as a solution
of tartaric acid and hydrogen peroxide, that does not etch the p type
AlGaAs etch stopping layer 5 but etches the p type AlGaAs second cladding
layer 6 and the p type GaAs first contact layer 7, whereby the ridge
structure can be formed with good reproducibility. Thereafter, as shown in
FIG. 2(d), the n type Al.sub.0.2 Ga.sub.0.8 As current blocking layer 8
doped with Si to a concentration equal to or larger than 1.times.10.sup.19
cm.sup.-3 is grown on both sides of the ridge structure to bury portions
of the p type AlGaAs second cladding layer 6 and the p type GaAs first
contact layer 7 which are removed by the etching. Since the insulating
film 13 serves as a mask during the crystal growth, the n type AlGaAs
current blocking layer 8 is not grown on the ridge structure.
In the step of FIG. 2(e), after removing the insulating film 13 by wet
etching or dry etching, the p type GaAs second contact layer 9 is grown on
the entire surface.
Finally, the n side electrode 10 is formed on the n type GaAs semiconductor
substrate 1 and the p side electrode 11 is formed on the p type GaAs
second contact layer 9, resulting in the semiconductor laser shown in FIG.
1.
In the semiconductor laser according to the first embodiment, the
quantum-well active layer 3 is formed so as to emit laser light having a
wavelength of 0.98 .mu.m. Because it is required to adjust the wavelength
of the laser light to the absorption wavelength of Er ions, i.e., 0.98
.mu.m, since the semiconductor laser of the first embodiment is used as a
light source for exciting an Er-doped fiber amplifier. To put it
concretely, as shown in FIG. 3, the active layer 3 is a multi quantum-well
layer comprising an Al.sub.0.2 Ga.sub.0.8 As barrier layer 31 having a
thickness of 20 nm, two In.sub.0.16 Ga.sub.0.84 As quantum-well layers 32
each having a thickness of 8 nm, and two Al.sub.0.2 Ga.sub.0.8 As guiding
layers each having a thickness of 30 nm. The AlGaAs barrier layer 31 is
sandwiched between the two InGaAs quantum-well layers 32, and further, the
InGaAs quantum-well layers 32 are sandwiched between the two AlGaAs
guiding layers
A description is given of a function of the n type AlGaAs current blocking
layer 8 doped with Si in a concentration equal to or larger than
1.times.10.sup.19 cm.sup.-3.
FIG. 4 is a diagram illustrating a molecule structure in the n type AlGaAs
current blocking layer doped with Si to a high concentration. FIG. 5 is a
graph showing Al dependency of an emission peak energy of a V.sub.III
-Si.sub.Ga complex in Al.sub.x Ga.sub.1-x As having an Si concentration of
about 1.times.10.sup.19 cm.sup.-3.
In FIG. 4, reference numeral 83 designates arsenic (As) as a Group V
element, numeral 84 designates gallium (Ga) as a Group III element,
numeral 85 designates aluminium (Al) as a Group III element, numeral 81
designates a vacancy of the Group III element site, and numeral 82
designates silicon (Si) as a Group IV element which is put into the
gallium site.
When AlGaAs is doped with Si to a high concentration, the silicon 82 put
into the gallium site and the vacancy 81 of the Group III element site in
the crystal electrically attract each other to produce a V.sub.III
-Si.sub.Ga complex C. The complexes C form a broad deep level in a
0.9.about.1.2 .mu.m wavelength band and absorb light having a wavelength
of 0.9.about.1.2 .mu.m. The light absorption by the complexes C is
strongly promoted when the Si concentration is equal to or larger than
1.times.10.sup.19 cm.sup.-3.
These complexes C are produced also when GaAs is doped with Si to a high
concentration. When Al is present, however, it is probable that the
gallium 84 of the Group III element site moves to produce the vacancy 81
therein. Therefore, in the Si-doped AlGaAs, the V.sub.III -Si.sub.Ga
complexes C are produced more easily and in larger quantities than those
produced in the Si-doped GaAs, whereby the absorption of the light having
a wavelength of 0.9.about.1.2 .mu.m is promoted more by the complexes C.
A description is given of the Al dependency of the emission peak energy of
the V.sub.III -Si.sub.Ga complex C in Al.sub.x Ga.sub.1-x As having an Si
concentration of 3.times.10.sup.18 cm.sup.-3 .about.1.times.10.sup.19
cm.sup.-3.
The graph shown in FIG. 5 is disclosed in Japanese Journal of Applied
Physics, Vol.61, No.9, pp.4603, 1987, T. Oh-hori, et al. The ordinate
represents an emission peak energy of Si-doped Al.sub.x Ga.sub.1-x As, and
the abscissa represents an Al composition ratio x of the Si-doped Al.sub.x
Ga.sub.1-x As.
As shown in FIG. 5, the emission peak energy takes a minimum value of 1.21
eV when Al is not present, i.e., when GaAs is doped with Si, and the peak
energy increases as the Al composition ratio x is increased. When the Al
composition ratio x is 0.11, the emission peak energy agrees with an
emission peak energy of light having a wavelength of 0.98 .mu.m. Since the
absorption peak energy is larger than the emission peak energy by
0.1.about.0.3 eV, an absorption peak energy of the Si-doped Al.sub.x
Ga.sub.1-x As agrees with that of the light having a wavelength of 0.98
.mu.m when the Al composition ratio x is about 0.2. Therefore, when the
Al.sub.x Ga.sub.1-x As having the Al composition ratio x of about 0.2 is
doped with Si to a high concentration, the absorption of the light having
a wavelength of 0.98 .mu.m is promoted, as compared with when Al is not
present.
The Al composition ratio of the AlGaAs current blocking layer 8 is larger
than those of the AlGaAs cladding layers 2, 4 and 6, the refractive index
of the current blocking layer 8 is smaller than those of the cladding
layers 2, 4, and 6, and the semiconductor laser does not include a loss
guide type structure. Therefore, it is required that the AlGaAs current
blocking layer 8 have an Al composition ratio x smaller than those of the
AlGaAs cladding layers 2, 4, and 6. In addition, in order to absorb the
laser light having a wavelength of 0.98 .mu.m, it is desirable to use
Al.sub.x Ga.sub.1-x As having an Al composition ratio x larger than 0 and
equal to or smaller than 0.3 in the current blocking layer 8. Especially,
Al.sub.x Ga.sub.1-x As having an Al composition ratio x of about 0.2 is
the most suitable therefor.
In order to form a deep level in the n type AlGaAs current blocking layer
8, the current blocking layer 8 may be doped with Si to a concentration
equal to or larger than 1.times.10.sup.19 cm.sup.-3. However, the Si
concentration is limited to about 1.times.10.sup.20 cm.sup.-3 considering
the solubility of Si in the crystal.
As described above, according to the semiconductor laser of the first
embodiment, the laser light having a wavelength of 0.98 .mu.m, which light
is emitted from the active layer 3 just below the ridge waveguide 12, is
absorbed by the V.sub.III -Si.sub.Ga complexes C in the Si-doped n type
AlGaAs current blocking layer 8. The concentration of the V.sub.III
-Si.sub.Ga complexes C increases with an increase in the Si concentration.
That is, the current blocking layer 8 in the semiconductor laser according
to the first embodiment serves as an absorption layer which absorbs the
laser light having a wavelength of 0.98 .mu.m. In this case, the waveguide
structure in the semiconductor laser is a loss guide type structure that
is different from the refractive index type structure as in the prior art
semiconductor laser shown in FIG. 8. In the semiconductor laser including
the loss guide type structure, the absorption of the laser light makes the
waveguide loss a little larger than that in the refractive index type
structure, makes the threshold current larger, and makes the quantum
efficiency lower. However, the absorption of the laser light by the
current blocking layer 8 is promoted more at the end portions than at the
center portion of the ridge waveguide 12, whereby the gains of the
higher-order modes having the peaks at the end portions of the ridge
waveguide are reduced more than the gains in the prior art semiconductor
laser including the refractive index type structure shown in FIG. 8.
Therefore, according to this semiconductor laser of the first embodiment,
even when the waveguide width Wb is broadened from 1.about.1.5 .mu.m to
2.about.4 .mu.m, a unimodal laser beam having a fundamental transverse
mode is produced stably. Further, when the waveguide width Wb is 2.about.4
.mu.m, operating current density at a constant light output is reduced, as
compared with when the width Wb is 1.about.1.5 .mu.m, and optical density
at the facet is reduced. Accordingly, reliability of the semiconductor
laser is significantly improved. Furthermore, since the waveguide width Wb
is broadened, it is possible to suppress the influence of variation of the
waveguide width Wb due to instability of the etching on variation of the
half-power angular width of the horizontal transverse mode. Consequently,
uniformity of device characteristics of the semiconductor laser is
improved.
In addition, the semiconductor laser according to the first embodiment
includes the current blocking layer 8 comprising Al.sub.0.2 Ga.sub.0.8 As
which is doped with Si to a concentration equal to or larger than
1.times.10.sup.19 cm.sup.-3. Therefore, unlike the prior art semiconductor
laser having the narrow absorption band as disclosed in Patent Application
7-178759, the broad deep level is formed in a 0.9.about.1.2 .mu.m
wavelength band by the V.sub.III -Si.sub.Ga complexes C which are produced
in the current blocking layer 8. In addition, since the current blocking
layer 8 comprises AlGaAs in the semiconductor laser of the first
embodiment, the absorption of the light having a wavelength of
0.9.about.1.2 .mu.m is promoted more than when the current blocking layer
comprises GaAs as in the prior art semiconductor laser shown in figure 11.
Therefore, the semiconductor laser according to the first embodiment of the
invention reliably suppresses the generation of the higher-order modes for
the light having a wavelength of 0.98 .mu.m and stably produces the laser
light having a fundamental mode. Consequently, an excellent semiconductor
laser for exciting an Er-doped fiber amplifier in a 0.98 .mu.m wavelength
band is obtained.
While the ridge waveguide 12 is formed by the selective etching using the
etch stopping layer 5, etching depth control may be carried solely by
controlling etching time, thereby forming the ridge waveguide 12, without
employing the etch stopping layer 5.
[Embodiment 2]
FIG. 6 is a cross-sectional view illustrating an SAS (self-aligned
structure) type semiconductor laser according to a second embodiment of
the present invention. This semiconductor laser of the second embodiment
is used as a light source for exciting an Er-doped fiber amplifier. In the
figure, reference numeral 1 designates an n type GaAs semiconductor
substrate. Ann type AlGaAs cladding layer 2 comprising Al.sub.0.5
Ga.sub.0.5 As is disposed on the n type GaAs semiconductor substrate 1. A
quantum-well active layer 3 comprising undoped InGaAs is disposed on the n
type AlGaAs cladding layer 2. A p type AlGaAs first cladding layer 4
comprising Al.sub.0.5 Ga.sub.0.5 As is disposed on the quantum-well active
layer 3. N type AlGaAs current blocking layers 16 are disposed on the p
type AlGaAs first cladding layer 4. The n type AlGaAs current blocking
layers 16 comprise Al.sub.0.2 Ga.sub.0.8 As doped with Si to a
concentration equal to or larger than 1.times.10.sup.19 cm.sup.-3. A p
type AlGaAs second cladding layer 6 comprising Al.sub.0.5 Ga.sub.0.5 As is
disposed on the n type AlGaAs current blocking layers 16 and on a portion
of the p type AlGaAs first cladding layer 4 where the current blocking
layer 16 is absent. Reference numeral 18 designates a waveguide, and the
waveguide 18 has a width Wb in a range of 2.about.4 .mu.m at a boundary
between the waveguide and the p type AlGaAs cladding layer 4. A p type
GaAs contact layer 17 is disposed on the p type AlGaAs second cladding
layer 6. A p side electrode 10 is disposed on a rear surface of the n type
GaAs semiconductor substrate 1, and an n side electrode 11 is disposed on
the p type GaAs contact layer 17.
The semiconductor laser according to the second embodiment differs from the
prior art semiconductor laser in that the current blocking layer is the n
type AlGaAs layer comprising Al.sub.0.2 Ga.sub.0.8 As having an Al
composition ratio x of 0.2 which is doped with Si to a concentration equal
to or larger than 1.times.10.sup.19 cm.sup.-3. The waveguide structure in
this semiconductor laser is a loss guide type structure that is different
from the refractive index type structure as in the prior art semiconductor
laser shown in FIG. 8.
A description is given of the fabricating method.
FIGS. 7(a)-7(e) are cross-sectional views illustrating process steps in the
fabricating method. Initially, as shown in FIG. 7(a), the n type AlGaAs
cladding layer 2, the quantum-well active layer 3, the p type AlGaAs first
cladding layer 4, and the n type AlGaAs current blocking layer 16
comprising Al.sub.0.2 Ga.sub.0.8 As having an Al composition ratio of 0.2
which is doped with Si to a concentration equal to or larger than
1.times.10.sup.19 cm.sup.-3 are successively epitaxially grown on the n
type GaAs semiconductor substrate 1, preferably by MOCVD. The growth of
the layers, except the active layer 3, is carried out under the conditions
of a growth temperature of about 700.degree. C., a V/III ratio of 200, and
a growth speed of 1 .mu.m/h.
Next, as shown in FIG. 7(b), an insulating film having a stripe-shaped
aperture is deposited on the n type AlGaAs current blocking layer 16. In
the step of figure 7(c), using the insulating film 130 as a mask, the n
type AlGaAs current blocking layer 16 is selectively etched until a
surface of the p type AlGaAs first cladding layer 4 is exposed, thereby
forming a stripe groove in the n type AlGaAs current blocking layer 16.
Thereafter, as shown in FIG. 7(d), after removing the insulating film 130
by etching, the p type AlGaAs second cladding layer 6 is grown on the
entire surface of the n type AlGaAs current blocking layer 16 and on the
exposed surface of the p type AlGaAs first cladding layer 4 to bury the
stripe groove in the n type AlGaAs current blocking layer 16.
Subsequently, as shown in FIG. 7(e), the p type GaAs contact layer 17 is
grown on the p type AlGaAs second cladding layer 6.
Finally, the n side electrode 10 is formed on the n type GaAs semiconductor
substrate 1 and the p side electrode 11 is formed on the p type GaAs
contact layer 17, preferably by vacuum evaporation, resulting in the SAS
type semiconductor laser shown in FIG. 6. The semiconductor laser
according to the second embodiment is used as a light source for exciting
an Er-doped fiber amplifier. Therefore, the quantum-well active layer 3 is
formed so as to emit laser light having a wavelength of 0.98 .mu.m as in
the first embodiment of the invention (refer to FIG. 3).
The semiconductor laser of the second embodiment has the same function and
effect as those of the ridge type semiconductor laser of the first
embodiment. More specifically, the V.sub.III -Si.sub.Ga complexes C are
produced in the n type AlGaAs current blocking layer 16, and the complexes
C form the broad deep level absorbing light having a wavelength of
0.9.about.1.2 .mu.m. Since the current blocking layer 16 comprises AlGaAs,
the absorption of the light having a wavelength of 0.9.about.1.2 .mu.m is
promoted more than when the current blocking layer comprises GaAs. As the
result, the absorption of the laser light emitted from the active layer 3
just below the waveguide 18 by the current blocking layer 16 is promoted
more at the end portions than at the center portion of the waveguide 18,
whereby the gains of the higher-order modes having the peaks at the end
portions of the waveguide 18 are reduced. Therefore, even when the
waveguide width Wb is broadened, the semiconductor laser according to the
second embodiment of the invention reliably suppresses the generation of
the higher-order modes for the light having a wavelength of 0.98 .mu.m and
stably produces the laser light having a fundamental mode. Consequently,
an excellent semiconductor laser for exciting an Er-doped fiber amplifier
in a 0.98 .mu.m wavelength band is obtained.
In the second embodiment of the invention, the etching depth control is
carried out solely by controlling etching time, thereby forming the stripe
groove, without employing the etch stopping layer 5 as in the first
embodiment. However, the stripe groove may be formed by forming a p type
AlGaAs etch stopping layer on the p type AlGaAs first cladding layer 4,
and carrying selective etching.
The semiconductor laser according to the present invention is not limited
to a light source for exciting an Er-doped fiber amplifier as in the first
and second embodiments. For example, if the quantum-well active layer is
formed so as to emit laser light having a wavelength of 1.02 .mu.m, the
semiconductor laser may be used as a light source for exciting a Pr-doped
fiber amplifier.
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